The present invention relates to an optical sensor which can be used as, for example, an image sensor, and more particularly to an optical sensor provided with an amorphous film, functioning as a photoconductive film.
Amorphous semiconductor film, a typical example of which is amorphous silicon film, can be deposited on a substrate by means of glow discharge of a gas, such as SiH.sub.4, or a combination of SiH.sub.4 with H.sub.2, PH.sub.3, B.sub.2 H.sub.6 and/or CH.sub.4. Further, it can be formed on a large surface area of the substrate. Due to these advantageous features, amorphous semiconductor film has attracted much attention in the art, since it can be used as a photoelectric conversion film in image sensors.
One of the various known optical sensors using amorphous semiconductor films has an electrode placed in ohmic contact with the amorphous semiconductor film. When the film is illuminated, it undergoes photoconduction, and its resistance changes. As a result, the film generates a photo-current, which is supplied as a signal from the electrode. The semiconductor amorphous film and the signal electrode form a photoelectric transducer element, usually called a "cell", of a photosensor. FIG. 1 is a graph illustrating the relationship between the voltage applied to the cell and the photoelectric current generated by the cell, as the amount of light irradiated to the cell is varied. As this figure shows, the greater the amount of light, the higher the resistance of the amorphous semiconductor film. FIG. 2 illustrates the relationship between the luminance on the cell surface and the photoelectric current generated by the cell. As is evident from FIG. 2, the photoelectric current increases as the luminance increases. Hence, a predetermined electric current can be obtained by applying an appropriate voltage to the cell.
It has been ascertained that the longer an amorphous semiconductor film is exposed to light, the higher the resistance the film will have, and hence, the smaller the photoelectric current will become. This phenomenon is known in the as Staebler and Wronski effect. That is, the photo-current gradually decreases as the film is continuously illuminated, even if the luminance (E) remains unchanged, as can be understood from the graph of FIG. 3. The decrease of the photoelectric current, due to this effect, is prominent in proportion to the luminance (E). The amorphous semiconductor film has its photosensitivity sharply reduced in a relatively short time. Therefore, the film cannot be practically used in an optical sensor.